Machining Crankshaft

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

The only real answer to making by ordinary turning methods a crankshaft too flexible to be self supporting lies in making a suitable support jig .

 

At simplest the jig need only consist of an arrangement of clamping plate to stiifen all the throws not currently being worked on and leaving only one journal unsupported .

 

Far better though is to make a proper wrap around jig with a hole in it and removable support pins and clamp screws such that all of the headstock end of the crankshaft is enveloped in the jig and just the currently being machined journal is sticking out (plus any previously machined journals of course but these don’t matter ).

 

There are two versions of the wrap around jig :

 

One with an axial hole which is set true for al the main journals and then reset eccentric for all the throw journals .

 

One which is in two parts with an eccentric main hole and then a sleeve with an eccentric hole to take the crankshaft . By setting the inner sleeve and the crankshaft itself in different ways the crankshaft can be set up to run true on the main journals or true on the throw journals but the main body of the jig does not have to be reset .

 

There are advantages in using round bar to make crankshaft with these jigs but round bar is by no means essential .

 

There is actually a third variant of the wrap around jig which is useful where there is only one throw – the bore in the jig body is itself made eccentric such that turning the crankshaft blank around in the eccentric hole sets either main or throw journals to run true .

 

MW

[JasonBModerator @jasonb]

Or just clamp one end in a keats angle plate and move that about on your faceplate until the journal is true, this gives support right upto the web and very little overhang.

 

J

[Sub MandrelParticipant @submandrel]

I’ve seen Jason’s partiing tool before – recommended by LBSC, and I can confirm it works.

 

Neil

[John BentleyParticipant @johnbentley76104]

John (Bridge) didn’t actually say that his crankshaft was unusually hard and in fact mentioned that he had already removed the surplus metal at the weld. It wouldn’t surprise me if he has finished the job successfully by now.

 

I have not found the Stuart crankshafts particularly flimsy while turning them normally. They cut very well without distortion in either my Peatol or my Chinese mini lathe. Similar freelance versions (up to three throws) made from solid bar have turned out equally well for me. I didn’t have any experience when I started out and haven’t had any trouble at all with conventional crankshaft designs like these. Success is probably the result of no more than using sharp tools at proper height with gentle cuts at sensible speed.

 

Jason’s recommendation of LSBC’s tool looks like a good one and I will be trying it out next time.

 

John

 

[JOHN BRIDGE 1Participant @johnbridge1]

Posted by John Bentley on 06/02/2012 21:59:13:

John (Bridge) didn’t actually say that his crankshaft was unusually hard and in fact mentioned that he had already removed the surplus metal at the weld. It wouldn’t surprise me if he has finished the job successfully by now.

 

I have not found the Stuart crankshafts particularly flimsy while turning them normally. They cut very well without distortion in either my Peatol or my Chinese mini lathe. Similar freelance versions (up to three throws) made from solid bar have turned out equally well for me. I didn’t have any experience when I started out and haven’t had any trouble at all with conventional crankshaft designs like these. Success is probably the result of no more than using sharp tools at proper height with gentle cuts at sensible speed.

 

Jason’s recommendation of LSBC’s tool looks like a good one and I will be trying it out next time.

 

John

This is correct John it was not unduly hard and I had removed the surplus, I have made a tool similar to the pic. and it is quite easy now to machine this journal it is now quite smooth but is still in the 3 jaw and revolving center in the tailstock there is still plenty of meat on the journal and I now intend to transfer to between centers and finish off this shaft, I am now quite confidant that I can now do the job. Like you I find the shaft quite sturdy and I do not intend to use any stengthening whilst turning.

John

[JOHN BRIDGE 1Participant @johnbridge1]

JasonB is the total width of that blade 3/32″, I would like to give it a try, what sort of grinder do you use to do the fine shaping, my grinder is a clarke 6″ which seems a little large for this job.

John

[JasonBModerator @jasonb]

You can either do it with a parting blade like the Eclipse ones which are nice and deep but they lack side clearance so either set them up dead true with a DTI or grind from a HSS blank but where the neck is narrow is a weak point.

 

Width would depend on the journal width buy I would say something like 1/3 of the total. And as you are only taking a few thou a pass with this tool us ealternate left & right hand tools to do most of the work.

 

I only have a 6″ bench grinder and a clarke belt/disc sander. a diamond stone will help with refining teh profile

 

I actually took the tool profile from Malcom Frosts (the M of MJ Engineering) traction engine articles no LBSC but guess that is where he got it from.

 

J

[KWILParticipant @kwil]

Jason has asked me to post this photo of the crank journal tool using round inserts. Setting is straight forward as a ground tool, if it needs sharpening, just rotate the inserts!

 

 

I hope you find it interesting

 

K

Edited By KWIL on 09/02/2012 21:06:31

[JOHN BRIDGE 1Participant @johnbridge1]

Thanks Kwil, It looks like a very useful tool if I ever get to do anything useful with this lathe of mine.

John

[JasonBModerator @jasonb]

I came accross this thread which shows a tool ground like the LBSC shape, may give you a better idea of how its done

 

J

 

PS Thanks for adding the photo KWIL.

[Sub MandrelParticipant @submandrel]

Wow Kwil, either those are very diddy inserts or that’s a meaty crankshaft 3 or 4″ Traction engine?

 

I like the idea of grinding crankshaft journals. A set up using my rotary table on my grinder appeals, but I’d need a different shape wheel. With a diamond wheel a mirror finish could be assured.

 

Neil

 

[KWILParticipant @kwil]

Neil,

 

They are 5mm diameter and it is a 4″ Traction engine, throw is 1.5″. The crankshaft was left +0.0005″ for grinding/lapping.

 

K

[Sub MandrelParticipant @submandrel]

Thanks Kwil,

 

Canme across a Tubal Cain crankshaft turning tool today in his Sirius series. HSS with teh double lobed end, but like your it is parallel, not tapered like a parting tool.

 

Neil

[DustyParticipant @dusty]

It has long been known that carbon steel tools can be made with a much keener edge than H.S.S. tools. There is however a downside in that they do not maintain this edge very well, basically they wear, as they wear the the edge is lost and finish and dimensional stability of the workpiece is lost (it gets bigger). This is especially true of mild steel EN3( I would not attempt anything harder) and some of the forged crankshafts we use. If you do use Carbon Steel tools, speed must be reduced and the tool flooded with cutting oil. The other problem with Carbon Steel tools is they can easily loose their hardness when grinding by overheating.

I make my bifurcated tools from parting tool blades and put the groove into the front of the tool with a Dremel/Proxxon and a cut of wheel.

[JOHN BRIDGE 1Participant @johnbridge1]

Dusty, Thanks for this post I was contemplating using a Dremel to cut the groove as I could see no way of doing it with my 6″ clarke.

John

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